U.S. patent number 4,259,392 [Application Number 05/916,430] was granted by the patent office on 1981-03-31 for multilayer magnetic recording medium.
This patent grant is currently assigned to Fuji Photo Film Co., Ltd.. Invention is credited to Masaaki Suzuki.
United States Patent |
4,259,392 |
Suzuki |
March 31, 1981 |
Multilayer magnetic recording medium
Abstract
A magnetic recording tape comprising a non-magnetic support
having thereon a dual layer magnetic recording coating comprising
ferromagnetic particles dispersed in a binder, wherein the fine
ferromagnetic particles of the outer layer are a mixture of two or
more kinds of fine ferromagnetic particles having two or more peaks
in the coercive force distribution and said particles containing at
least one kind of fine ferromagnetic alloy particles and the fine
ferromagnetic particles of the inner layer being ferromagnetic iron
oxide particles having peaks in the coercive force distribution
lower than the minimum peaks in the coercive force distribution of
the ferromagnetic particles of the outer layer, with the thickness
of the outer layer not being greater than the thickness of the
inner layer.
Inventors: |
Suzuki; Masaaki (Odawara,
JP) |
Assignee: |
Fuji Photo Film Co., Ltd.
(Minami-ashigara, JP)
|
Family
ID: |
13433533 |
Appl.
No.: |
05/916,430 |
Filed: |
June 16, 1978 |
Foreign Application Priority Data
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Jun 16, 1977 [JP] |
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52/70508 |
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Current U.S.
Class: |
428/212; 360/135;
360/136; 427/131; 428/336; 428/339; 428/839.3; 428/900;
G9B/5.278 |
Current CPC
Class: |
G11B
5/716 (20130101); Y10T 428/265 (20150115); Y10S
428/90 (20130101); Y10T 428/269 (20150115); Y10T
428/24942 (20150115) |
Current International
Class: |
G11B
5/716 (20060101); H01F 010/02 () |
Field of
Search: |
;428/539,336,900,339,329,212 ;360/134,135,136 ;427/131,128 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1909155 |
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Jul 1973 |
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DE |
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2507975 |
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Sep 1975 |
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DE |
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2615961 |
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Oct 1976 |
|
DE |
|
Primary Examiner: Silverman; Stanley S.
Attorney, Agent or Firm: Sughrue, Rothwell, Mion, Zinn and
Macpeak
Claims
What is claimed is:
1. A multilayer magnetic recording medium comprising a non-magnetic
support having thereon two magnetic recording layers, each
comprising fine ferromagnetic particles dispersed in a binder, said
fine ferromagnetic particles of the uppermost magnetic recording
layer being a mixture of 2 or more kinds of fine ferromagnetic
particles having two or more peaks in the coercive force
distribution and said ferromagnetic alloy particles, wherein when
the peak values in the coercive force distribution of the fine
ferromagnetic particles in the outermost magnetic recording layer
are Hc.sub.2, Hc.sub.3, Hc.sub.4, . . . Hc.sub.n, the mixing ratio
by weight of the fine ferromagnetic particles is (0.4/n) to
(1.6/n), said fine ferromagnetic particles of the innermost
magnetic recording layer being fine ferromagnetic iron oxide
particles having peaks in the coercive force distribution lower
than the minimum value of the peaks of the ferromagnetic particles
of the outermost magnetic recording layer, and wherein the peak
values in the coercive force distribution of the fine ferromagnetic
particles in the outermost magnetic recording layer are Hc.sub.2,
Hc.sub.3, . . . Hc.sub.n and when the peak value is Hc.sub.1 in the
coercive force distribution of the fine ferromagnetic particles in
the innermost magnetic recording layer, these peak values are in
the relationship
the thickness of the outermost magnetic recording layer being not
greater than the thickness of the inner magnetic recording
layer.
2. The multilayer magnetic recording medium of claim 1, wherein the
peak value Hc.sub.1 in the coercive force distribution of the fine
ferromagnetic particles in the innermost magnetic recording layer
is 200 oe<Hc.sub.1 <470 oe and the peak values Hc.sub.2 and
Hc.sub.3 in the coercive force distribution of the fine
ferromagnetic particles in the outermost magnetic recording layer
are 315 oe<Hc.sub.2 <900 oe and 460 oe<Hc.sub.2 <1,320
oe, respectively.
3. The multilayer magnetic recording medium of claim 1, wherein the
peak value Hc.sub.1 in the coercive force distribution of the fine
ferromagnetic particles in the innermost magnetic recording layer
is 220 oe<Hc.sub.1 <520 oe and the peak values Hc.sub.2 and
Hc.sub.3 in the coercive force distribution of the fine
ferromagnetic particles in the outermost magnetic recording layer
are 345 oe<Hc.sub.2 <990 oe and 500 oe<Hc.sub.3 <1,450
oe, respectively.
4. The multilayer magnetic recording medium of claim 1, wherein the
weight ratio of the fine ferromagnetic particles having the peak
value of Hc.sub.2 and the fine ferromagnetic particles having the
peak value of Hc.sub.3 for the outermost magnetic recording layer
is about 4:1 to about 1:4.
5. The multilayer magnetic recording medium of claim 1, wherein the
thickness of the outermost magnetic recording layer is about 0.5 to
3.5 .mu.m, the thickness of the innermost magnetic recording layer
is about 2.5 to 15 .mu.m and the thickness of the outermost
magnetic recording layer is not greater than the thickness of the
innermost magnetic recording layer.
6. The multilayer magnetic recording medium of claim 5, wherein the
fine ferromagnetic particles in the outermost recording layer are
Fe-Co-Ni alloy and .gamma.-Fe.sub.2).sub.3 and the fine
ferromagnetic iron oxide particles in the innermost magnetic
recording layer are .gamma.-Fe.sub.2 O.sub.3.
7. The multilaryer magnetic recording medium of claim 5, wherein
the fine ferromagnetic particles in the outermost recording layer
are Fe-Co-Ni alloy and Co-FeOx and the fine ferromagnetic iron
oxide particles in the innermost magnetic recording layer are
.gamma.-Fe.sub.2 O.sub.3.
8. The multilayer magnetic recording medium of claim 1, wherein the
surface of the innermost magnetic recording layer has a mean
surface roughness of less than about 0.2 S due to a surface
smoothening treatment and the surface of the outermost magnetic
recording layer has a mean surface roughness of less than about 0.2
S due to a surface smoothening treatment.
9. The multilayer magnetic recording medium of claim 1, which is an
audio recording medium and which illustrates good linearity over a
broad frequency range and a broad dynamic range.
10. The multilayer magnetic recording medium of claim 1, wherein
the peak value Hc.sub.1 is 200 Oe<Hc.sub.1 <300 Oe and the
peak values of Hc.sub.2 and Hc.sub.3 are 315 Oe<Hc.sub.2 <560
Oe and 460 Oe<830 Oe, respectively.
11. The multilayer magnetic medium of claim 1, wherein the peak
value of Hc.sub.1 is 335 Oe<Hc.sub.1 <470 Oe and the peak
values Hc.sub.2 and Hc.sub.3 are 500 Oe<Hc.sub.2 <900 Oe and
740 Oe<Hc.sub.3 <1320 Oe, respectively.
12. The multilayer magnetic recording medium of claim 1, wherein
said outermost magnetic recording layer has a residual magnetic
flux density Br of at least 1,700 Gauss.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a multilayer magnetic recording medium
and, in particular, to a dual layer magnetic recording tape having
improved magnetic recording properties.
2. Description of the Prior Art
Recently, with the requirements of quality improvement and high
density of magnetic recording tapes in both open reel tapes and
cassette tapes, multilayer type magnetic recording tapes, that is,
magnetic recording tapes having two or more magnetic recording
layers, have been developed.
Multilayer magnetic recording tapes are described in, for example,
Japanese Patent Publication Nos. 2218/'62 and 23,678/'64; Japanese
Patent Application (OPI) Nos. 31,602/'72 (the term "OPI" as used
herein refers to a "published unexamined Japanese patent
application") (or U.S. Pat. No. 3,761,311); 31,804/'72; 31,907/'73
(or U.S. Pat. No. 3,775,170); and 31,804/'75; U.S. Pat. Nos.
2,643,130; 2,647,954; 2,041,901 and 3,676,217; and West German
Patents (DT-AS) Nos. 1,190,905 and 1,238,072.
In this case, an important problem is the setting of the operating
bias and equalization and the setting value is usually very near a
generally designated standard bias. Therefore, magnetic recording
tapes operable at this value are readily and most generally used
and thus are very advantageous since such magnetic recording tapes
do not require any specific bias setting and equalization.
As the easiest means for increasing the magnetic recording density,
increasing the coercive force of the ferromagnetic iron oxide used
for the magnetic recording tapes has been proposed. However, such
magnetic recording tapes are not generally compatible with the
above-described standard bias in using these tapes and in order to
obtain optimum characteristics for magnetic recording tapes, a
different operating bias and equalization setting for the magnetic
recording tapes must be used. That is, a standard bias, a chromium
dioxide bias, a multilayer magnetic tape bias, etc. are used.
Conventional dual layer magnetic recording tapes may be superior in
the output in the low frequency region but they require a specific
position in operating bias and equalization.
This is, for example, as shown in Table 1 below.
TABLE 1 ______________________________________ Type of Magnetic
Bias.sup.(1) Equalizer.sup.(2) Recording Tape (%) (.mu.sec.)
______________________________________ Low-Noise Type 100 120 Fe-Cr
Type.sup.(3) 130 35-50 CrO.sub.2 Type 160 70
______________________________________ .sup.(1) Low noise type
magnetic recording tape is shown as standard (100%). .sup.(2) The
time constant (.mu.sec.) of low noise type equalization is shown as
a standard (120 .mu.sec.). The time constant of a FeCr type
magnetic recording tape is usually 40-60% of the standard value and
that of a CrO.sub.2 type magnetic recording tape is usually 50-70%
of the standard value. .sup.(3) DUAD Ferri Chrome tape (registered
trade name, made by SONY Corp.), Scotch CLASSIC Cassette tape
(registered trade mark, made by 3M Co.), Ferrochrome tape
(registered trade name, made by BASF A.G.), and CARAT tape
(registered trade mark, made by AGFAGEVAERT N.V.) are used as
examples.
Accordingly, in order to use tape recorders, tape decks, etc.,
under the best conditions for a magnetic recording tape, the
devices must be equipped with the bias and equalization positions
as shown in Table 1 above.
Conventional low-noise type and CrO.sub.2 type magnetic recording
tapes which are known to have the best characteristics have, for
example, the fundamental properties of Sample No. 1 and Sample No.
2 shown in Table 4 below. Furthermore, multilayer magnetic
recording tapes developed for the purpose of further improving the
characteristics of magnetic recording tapes, Sample No. 1 and
Sample No. 2 have the fundamental properties of, for example,
Sample No. 3 and Sample No. 4 as illustrated in Table 4 shown
hereinafter.
In addition, the layer structure of a dual layer type magnetic
recording tape is illustrated in FIG. 1 of the accompanying
drawings as a schematic enlarged sectional view thereof, wherein an
inner magnetic recording layer 2 and an outer magnetic recording
layer 1 are formed on a non-magnetic support 3.
In Table 4, Sample No. 3 is the dual layer magnetic recording tape
prepared according to the description of Japanese Patent
Application (OPI) No. 51,908/'77 (corresponding to U.S. Pat. No.
4,075,384) and Sample No. 4 is also a dual layer magnetic recording
tape prepared in a similar manner except that the coercive force of
each magnetic recording layer thereof was 1.6 times higher than
that of each corresponding magnetic recording layer of Sample No.
3. As will be understood from Table 4, a dual layer magnetic
recording tape can be provided with excellent characteristics as
compared with single layer type magnetic recording tapes.
Also, multilayer (dual layer) magnetic recording tapes prepared for
the purpose of further improving the sensitivity in the high
frequency range for the frequency characteristics (FIG. 2 and FIG.
3) of Sample No. 3 and Sample No. 4 have the fundamental
characteristics of, for example, Sample No. 5 and Sample No. 6
shown in Table 4. As is shown in FIG. 2 and FIG. 3 of the
accompanying drawings, both Sample No. 5 and Sample No. 6 have
higher sensitivity in the high frequency range than those of Sample
No. 3 and Sample No. 4 but the sensitivity in the range of 2 KHz-6
KHz is low, which results in unbalanced sound. Furthermore, on
comparing Sample No. 3 with Sample No. 5 and Sample No. 4 with
Sample No. 6, it will be understood that Sample No. 5 and Sample
No. 6 have a higher harmonic distortion factor, a lower maximum
output level (MOL), a lower S/N ratio (signal to noise ratio), and
less dynamic range. Thus, when the difference in coercive force
between the inner magnetic recording layer and the outer magnetic
recording layer of a dual layer magnetic recording tape is
increased, the frequency characteristics may be increased but the
sound balance frequently becomes poor.
Moreover, in order to record and reproduce using a magnetic
recording head without reducing the magnetic characteristics of the
inner magnetic recording layer of a dual layer magnetic recording
tape, the outer magnetic recording layer thereof preferably is thin
and in order to obtain sufficient effects such as an improvement in
sensitivity in the high freqeuncy range by a thin outer magnetic
recording layer, it is desirable to increase the maximum residual
magnetic flux density of the magnetic recording layer.
Therefore, in using fine particles of chromium dioxide having a
comparatively high coercive force for the outer magnetic recording
layer of a dual layer magnetic recording tape, there are the
disadvantages that it is difficult to industrially produce chromium
dioxide particles having a coercive force of higher than about 700
oe and when the coercive force thereof is increased, the maximum
residual magnetic flux density is reduced. Also, the use of
chromium dioxide particles for the outer magnetic recording layer
is undesirable since the abrasion of the magnetic recording head is
severe and the life of the magnetic recording head is reduced.
Still further, since thermal demagnetization of a magnetic
recording layer composed of chromium dioxide particles occurs to a
great extent and the balance between the magnetic characteristics
of the outer magnetic recording layer and the magnetic
characteristics of the inner magnetic recording layer due to
changes in temperature is lost, the use of chromium dioxide
particles as a material for one magnetic recording layer of a
multilayer magnetic recording material is not very desirable.
SUMMARY OF THE INVENTION
The present invention provides a magnetic recording medium capable
of providing the best characteristics with an ordinary tape
recorder or tape deck unaccompanied by the above-described
disadvantages, having good linearity over broad frequency ranges
and broad dynamic range, i.e., noise to maximum output range, and
having good total balance and relates to a magnetic recording tape
having two magnetic recording layers on a support.
A first object of this invention is to provide a dual layer
magnetic recording medium having two magnetic recording layers and
capable of providing the best characteristics of a magnetic tape at
a conventional bias and/or equalization position such as, for
example, a low noise position and a CrO.sub.2 position without need
of a new tape bias selection position.
A second object of this invention is to provide a dual layer
magnetic recording medium with good linearity over broad frequency
ranges and a broad dynamic range and showing good overall
balance.
Use of a mixture of two kinds of ferromagnetic particles, each
having a difference coercive force, for the outer magnetic
recording layer for improving the various characteristics of Sample
No. 5 and Sample No. 6 described above was investigated and as the
result thereof, it has been found that these new samples have the
fundamental characteristics of Sample No. 7 and Sample No. 8 shown
in Table 5 hereinafter, that is, Sample No. 7 has higher frequency
characteristics than Sample No. 3 and the difficulties in Sample
No. 5 described above have been overcome in the sample.
That is, as the result of investigations on the manner of using a
mixture of two or more kinds of ferromagnetic particles, each
having a different coercive force, as materials for the outer
magnetic recording layer of a dual layer magnetic recording medium,
the present invention has been obtained.
Thus, according to this invention, there is provided a multilayer
magnetic recording medium comprising a non-magnetic support having
thereon two magnetic recording layers, each mainly composed of fine
ferromagnetic particles dispersed in a binder, the fine
ferromagnetic particles of the outer magnetic recording layer being
a mixture of two or more kinds of fine ferromagnetic particles
having two or more peaks in the coercive force distribution and the
particles containing at least one kind of ferromagnetic alloy
particles, the fine ferromagnetic particles of the inner magnetic
recording layer being fine ferromagnetic iron oxide particles
having peaks in the coercive force distribution lower than the
miminum of the peaks in the coercive force distribution of the
ferro-magnetic particles of the outer magnetic recording layer, and
the thickness of the outer magnetic recording layer being not
greater than the thickness of the inner magnetic recording
layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic enlarged cross sectional view showing the
layer structure of a multilayer magnetic recording tape.
FIG. 2 is a graph showing the frequency characteristics of a
low-noise type magnetic recording tape at 100% bias (a standard
tape is assumed to be flat at 0 dB).
FIG. 3 is a graph showing the frequency characteristics of a
CrO.sub.2 type magnetic recording tape at 160% bias (the bias value
of a standard tape at 100% bias is assumed to be flat at 0 dB).
FIGS. 4(a), (b), (c), and (d) are graphs showing examples of
coercive force distributions of ferromagnetic particles.
FIG. 5 is a graph showing B-H characteristics.
FIG. 6 is a graph showing the relationships between the operating
bias value of a magnetic recording tape and the thickness of the
magnetic recording layers.
FIG. 7 is a graph showing a bias curve.
DETAILED DESCRIPTION OF THE INVENTION
The coercive force distribution is, for example, shown by Curve 1
in FIG. 4 and the peak value (.mu.) is shown by the value of .mu.
as shown in FIG. 4(a). As shown in, for example, FIG. 4(b), when
ferromagnetic particles A (Curve 2) are mixed with ferromagnetic
particles B (Curve 3), the coercive force distribution shows only
one clear peak value (.mu.B) as shown by Curve 4. However, as shown
in FIG. 4(c), when the mixing ratio of ferromagnetic particles A
(Curve 5) and ferromagnetic particle B (Curve 6) is changed, the
coercive force distribution clearly shows two peaks (.mu.A and
.mu.B) as shown by Curve 7 and hence it can be determined before
kneading or coating the ferromagnetic particles whether the
ferromagnetic particles are composed of the same kind of
ferro-magnetic particles or different kinds of ferromagnetic
particles. Furthermore, as shown in FIG. 4(d), when the composition
of ferromagnetic particles C (Curve 8) is the same as that of
ferromagnetic particles D (Curve 9) (for example, both are acicular
gamma-Fe.sub.2 O.sub.3 particles) and the .mu. values thereof are
close to each other, only one peak appears when they are mixed
together as shown in Curve 10. Hence it is difficult to distinguish
ferromagnetic particles C from magnetic particles D of the mixture.
Therefore, the coercive force distribution of a mixture of
ferromagnetic particles having two or more peaks in the coercive
force distribution used for the outer magnetic recording layer in
this invention is the coercive force distribution as shown in FIG.
4(c).
The results of the investigations leading to this invention will be
described in greater detail. That is, when the B-H characteristics
of single layer magnetic recording tapes are almost the same except
that the H-axis of the magnetic field differs in proportion to the
coercive force (as shown in FIG. 5), the operation bias value
determined by the thickness of magnetic recording tape and the
coercive force thereof has the value shown in FIG. 6. In this case,
when the thickness of the magnetic recording tape is increased, the
bias curve (as shown in FIG. 7) tends to become broader, the
sensitivity tends to increase, the frequency charcteristics tend to
decrease, the harmonic distortion factor tends to be reduced, and
the maximum output level tends to increase. Also, when the coercive
force increases, the operation bias value is increased, the
sensitivity is reduced, and the frequency characteristics are
increased. Therefore, for a low-noise type magnetic recording tape,
a reproducing equalization (Table 1) in which the time constant at
the high frequency side is reduced is generally used at the
CrO.sub.2 position. Therefore, on considering the balance of the
various characteristics, single layer magnetic recording tapes are
prepared as, for example, Sample No. 1 and Sample No. 2 (Table
4).
When a dual layer magnetic recording tape is prepared as a
low-noise type magnetic recording tape in such manner that the
outer magnetic recording layer (FIG. 6-1) has a thickness of 2.4
.mu.m on the curve of 100% in FIG. 6 and the inner magnetic
recording layer (FIG. 6-2) has a thickness of 3.6 .mu.m on the
curve of 80% in FIG. 6, a dual layer magnetic recording tape (FIG.
6-3) having an operation bias which is almost 100% is obtained. The
dual layer magnetic recording tape thus prepared has the
fundamental characteristics of Sample No. 3 (Table 4 and FIG. 2).
Also, when a dual layer magnetic recording tape (Table 4 and FIG.
2) of Sample No. 5 is prepared experimentally in such manner that
the outer magnetic recording layer has a thickness of 2.0 .mu.m on
the extension line of the 120% curve in FIG. 6 and the inner
magnetic recording layer (FIG. 6-5) has a thickness of 4.0 .mu.m on
the 75% curve in FIG. 6, a dual layer magnetic recording tape of an
operation bias of almost 100% is obtained (FIG. 6-3).
However, when the operation bias value of the outer magnetic
recording layer is much higher than the operation bias to be used
for the magnetic recording tape, the frequency characteristics
extend to the high frequency range but it has been confirmed that
in this case, the frequency of the characteristics at 2 KHz to 6
KHz is low (see, Sample No. 3 and No. 5 in FIG. 2), the harmonic
distortion factor is high, the maximum output level is low, the S/N
ratio is decreased, and the dynamic range is decreased (Table 4).
Thus, a magnetic recording tape having an apparent coercive force
almost the same as that of Sample No. 5 is prepared as Sample No.
7. Then, as shown in Table 5, the dual layer magnetic recording
tape of Sample No. 7 thus prepared has various characteristics
which are better than those of Sample No. 5 and having an excellent
total balance superior to that of Sample No. 3 (see, Sample Nos. 3,
5 and 7 of FIG. 2).
When a dual layer magnetic recording tape for the CrO.sub.2
position is prepared in this invention, the magnetic recording tape
of Sample No. 8 having better characteristics than those of Sample
No. 6 and having good balance superior to that of Sample No. 4 is
obtained (see Samples Nos. 4, 6 and 8 of Table 5 and FIG. 3).
It is well known to those skilled in the art that when the surface
property of a magnetic recording tape is improved and the
squareness ratio of the B-H characteristics and the saturation
property of the initial magnetization curve are improved, the
distribution of the operation bias in FIG. 6 shifts to the left
side in FIG. 6. These improvements are effective in this invention
for obtaining similar effects. Accordingly, in preparing each of a
low-noise type magnetic recording tape and a CrO.sub.2 type
magnetic recording tape, unavoidably in the distribution of the
operation bias shown in FIG. 6, some change in the coercive force
axis occurs. Also, it is known that the inclination of the
distribution curve of the operation bias shown in FIG. 6 changes
depending on the length and type of the gap of the magnetic
recording head employed.
In producing the dual layer magnetic recording medium in this
invention, as can be seen from FIG. 6, in order to increase the
coercive force of the outer magnetic recording layer and to extend
to frequency range to the high frequency side, it is preferred for
the outer magnetic recording layer be comparatively thinner. It is
preferred for the thickness of the outer magnetic recording layer
to at least be thinner than that of the inner magnetic recording
layer.
In the dual layer magnetic recording tape of this invention, it is
preferred for the outer magnetic recording layer to have a
thickness of from about 0.5 .mu.m to about 3.5 .mu.m, particularly
from about 1 .mu.m to about 3 .mu.m, for the inner magnetic
recording layer to have a thickness of from about 2.5 .mu.m to
about 15 .mu.m, particularly from about 3 .mu.m to about 10 .mu.m,
and for the thickness of the outer magnetic recording layer to be
the same as or thinner than the thickness of the inner magnetic
recording layer.
When ferromagnetic particles (1) having a coercive force (peak
value) of Hc.sub.1 are used for the inner magnetic recording layer
and a mixture of ferromagnetic particles (2) having a coercive
force (peak value) of Hc.sub.2 and ferromagnetic particles (3)
having a coercive force (peak value) of Hc.sub.3 is used for the
outer magnetic recording layer as a method of producing each of a
low noise type magnetic recording tape and a CrO.sub.2 type
magnetic recording tape in this invention, it is particularly
preferred for the values of Hc.sub.1, Hc.sub.2 and Hc.sub.3 to be
in the ranges shown in Table 2 below in relation to the thickness
of the outer layer of each magnetic recording tape.
it has now been found that when the weight ratio of the
ferromagnetic particles (2) to the ferromagnetic particles (3) is
from 4:1 to 1:4 (Hc.sub.2 /Hc.sub.3), the advantages of this
invention are obtained sufficiently. Also, it is preferred for the
relationship between the peak value (Hc.sub.1) of the coercive
force distribution of the inner layer described above and the peak
values (Hc.sub.2 and Hc.sub.3 ) of the coercive force distributions
of the outer layer to be Hc.sub.1 <Hc.sub.2 <Hc.sub.3.
The ranges of these Hc.sub.1, Hc.sub.2 and Hc.sub.3 values are
200<Hc.sub.1 <470, 315 oe<Hc.sub.2 <900 oe, 460
oe<Hc.sub.3 <1320 oe, and Hc.sub.1 <Hc.sub.2 <Hc.sub.3
; or 220 oe<Hc.sub.1 <520 oe, 345 oe<Hc.sub.2 <990 oe,
500 oe<Hc.sub.2 <1450 oe, and Hc.sub.1 <Hc.sub.2
<Hc.sub.3.
Particularly preferred ranges of these Hc.sub.1, Hc.sub.2, Hc.sub.3
values and the thickness (in .mu.m) of the outer magnetic recording
layer are shown in Table 2 below.
TABLE 2
__________________________________________________________________________
Thickness of Outer Magnetic Recording Low Noise Type* CrO.sub.2
Type* Layer Hc.sub.1 Hc.sub.2 Hc.sub.3 Hc.sub.1 Hc.sub.2 Hc.sub.3
__________________________________________________________________________
(.mu.m) (oe) (oe) (oe) (oe) (oe) (oe) 1.0 200-260 460-560 670-830
335-415 740-900 1080-1320 1.5 205-265 405-505 605-745 340-420
660-800 970-1190 2.0 210-270 370-450 550-670 345-425 580-720
870-1070 2.5 220-280 335-415 500-610 360-440 540-660 800-980 3.0
240-300 315-385 460-560 390-470 500-620 740-900 Overall Range
200-300 315-560 460-830 335-470 500-900 740-1320
__________________________________________________________________________
*See Table 1
In Table 2 described above, the weight ratio of the ferromagnetic
particles (2) to the ferromagnetic particles (3) (Hc.sub.2
/Hc.sub.3) is from 4:1 to 1:4, preferably from 3:2 to 2:3 as
described above.
In the present invention, the peak value in the coercive force
distribution of the outer magnetic recording layer may be at least
2 and when the peak value is higher than 2, the order of the peak
values is Hc.sub.2, Hc.sub.3, Hc.sub.4, . . . , Hc.sub.n and in
this case, it is preferred for the relationship thereof and the
peak value (Hc.sub.1) in the coercive force distribution of the
inner magnetic recording layer in this case to be Hc.sub.1
<Hc.sub.2 <Hc.sub.3 <Hc.sub.4 <Hc.sub.5 < . . .
Hc.sub.n. In this case, the weight ratio of Hc.sub.2, Hc.sub.3,
Hc.sub.4, Hc.sub.5, . . . , Hc.sub.n is (0.4/n) to (1.6/n),
preferably (0.8/n) to (1.2/n) for Hc.sub.n. For example, when n is
3, the weight ratio is 13 to 53:13 to 53:13 to 53, preferably 26 to
40:26 to 40:26 to 40.
In addition, it is preferred for the maximum residual magnetic flux
densities Br of the outer magnetic recording layer and the inner
magnetic recording layer in the dual layer magnetic recording
medium of this invention to be in the range of from about 1,500
Gausses to 3,500 Gausses.
The ranges shown in Table 2 described above are the ranges for
application of the present invention to conventional low-noise type
and CrO.sub.2 type magnetic recording tapes for Philips cassettes
of a width of 3.81 mm with the best effect. Furthermore, the
invention can also be applied with similar effect to other type
magnetic recording tapes such as, for example, magnetic recording
tapes for open reel of a width of 1/4 inch, EL cassettes, and
mini-size cassettes (micro-cassettes) for high density recording
and the invention can be applied to still other magnetic recording
media with a similar effect. Therefore, it will be understood by
those skilled in the art that in the production of various magnetic
recording media, the numerical values shown in Table 2 can be
appropriately changed without departing from the spirit and the
scope of the present invention.
That is, as shown in Table 2, in producing a low-noise type dual
layer magnetic recording tape, it is necessary to use ferromagnetic
particles providing a peak value in the coercive force distribution
at 460 to 830 oe and in the case of producing a CrO.sub.2 type dual
layer magnetic recording tape, it is necessary to use ferromagnetic
particles providing a peak value in the coercive force distribution
at 740 to 1,320 oe. Therefore, it has been discovered that the use
of ferromagnetic alloy particles as the ferromagnetic particles of
high coercive force for the outer magnetic recording layer is most
preferred and by using such ferromagnetic alloy particles,the
above-described difficulties encountered in the case of using
chromium dioxide particles for the outer layer using conventional
techniques have all been overcome.
Also, it is preferred to use ferromagnetic iron oxide particles as
the ferromagnetic particles of comparatively low coercive force for
the inner magnetic recording layer of the dual layer magnetic
recording tape of this invention but it has been confirmed that, in
using ferromagnetic iron oxide particles in admixture with
ferromagnetic alloy particles for the inner magnetic recording
layer and, further, in using a mixture thereof for the outer
magnetic recording layer on an inner magnetic recording layer
containing ferromagnetic iron oxide particles, they have excellent
affinity, dispersibility, and physical properties and, in
particular, a synergistic action of the ferromagnetic properties,
which is an object of this invention, is ideally obtained.
In particular, it is preferred to use ferromagnetic alloy particles
as the ferromagnetic particles of high coercive force for the outer
magnetic recording layer of the dual layer magnetic recording
medium of this invention. Since ferromagnetic alloy particles have
a comparatively high coercive force and a high residual magnetic
flux density as shown in Table 3 hereinafter, high sensitivity is
obtained in using such ferromagnetic alloy particles and the use of
such ferromagnetic alloy particles is suitable for the purpose of
this invention.
It is preferred for ferromagnetic iron oxide particles to be used
as the ferromagnetic particles of comparatively low coercive force
for the inner magnetic recording layer of the dual layer magnetic
recording medium of this invention. Ferromagnetic iron oxide
particles have the advantages that particles having a comparatively
uniform coercive force can be easily obtained in a stable condition
and are inexpensive.
TABLE 3 ______________________________________ Chemical Composition
Description Coercive Force (Hc)
______________________________________ (oersted) .gamma.-Fe.sub.2
O.sub.3 Maghemite (.gamma.-hemite) Granular 100-200 .gamma.-Iron
Oxide Acicular 240-450 Co-.gamma.-Fe.sub.2 O.sub.3 Cobalt Ferrite
Granular 200-1000 Cobalt-Doped Acicular 300-1500 .gamma.-Iron Oxide
Fe.sub.3 O.sub.4 Magnetite Acicular 260-450 Co-Fe.sub.3 O.sub.4
Cobalt-Doped Acicular 320-2500 Magnetite Co-FeO.sub.x Beridox*
Acicular 500-1500 (x = 1.33-1.50) Avilyn** CrO.sub.2 Chromium
Dioxide Acicular 90-700 Fe-Co-(Ni) Alloy Chain-like or 200-1800
Acicular ______________________________________ *Trade name for a
Cocontaining Berthollide iron oxide, made by the Fuji Photo Film
Co., Ltd. **Trade name for a Cocontaining Berthollide iron oxide,
made by Tokyo Denki Kagaku Kogyo K.K.
The ferromagnetic iron oxides shown in Table 3 above are
ferromagnetic iron oxides represented by the general formula
FeO.sub.x wherein x is in the range 1.33.ltoreq..times..ltoreq.1.5,
that is, they are maghemite (.gamma.-Fe.sub.2 O.sub.3, x=1.50),
magnetite (Fe.sub.3 O.sub.4, x=1.33), and Berthollide compounds
thereof (FeO.sub.x, 1.33<x<1.50).
x in the above formulas is shown by the following relationship;
##EQU1## wherein A is the atomic percent of divalent iron.
B is the atomic percent of trivalent iron.
These ferromagnetic iron oxides are particularly preferred as the
materials for the inner magnetic recording layer of this
invention.
These ferromagnetic iron oxides may contain a divalent metal such
as Cr, Mn, Co, Ni, Cu and Zn and a suitable amount of such a
divalent metal is 0 to about 10 atomic percent to the iron
oxide.
The acicular ratio of the above described ferromagnetic iron oxide
particles is about 2:1 to 20:1, preferably higher than 5:1, and the
mean particle length is about 0.2 to 2.0 .mu.m.
Processes of producing these ferromagnetic iron oxides are
described in, for example, Japanese Patent Publication Nos.
5515/'61; 4825/'62; 5009/'64; 10,307/'64; 6538/'66; 6113/'67;
20,381/'67; 14,090/'69; 14,934/'70; 18,372/'70; 28,466/'71;
21,212/'72; 27,719/'72; 39,477/'72; 40,758/'72; 22,269/'73;
22,270/'73; 22,915/'73; 27,200/'73; 39,639/'73; 44,040/'73; and
15,757/'74; Japanese Patent Application (OPI) Nos. 22,707/'72;
8496/'74; 4199/'74; 41,299/'74 (or West German Patent Application
(DT-OS) Nos. 2,221,264); 41,300/'74 (or West German Patent
Application (DT-OS) Nos. 2,221,218); and 69,588/'74 (or West German
Patent Application (DT-OS) No. 2,243,231); West German Patent
Application (DT-OS) No. 2,022,013; and U.S. Pat. Nos. 3,075,919;
3,398,014; 3,573,980; and 3,725,126.
Examples of ferromagnetic chromium dioxides as described above are
CrO.sub.2 particles and CrO.sub.2 particles containing 0 to about
20% by weight of a metal such as Na, K, Ti, V, Mn, Fe, Co, Ni, Te,
Ru, Sn, Ce, Pb, etc., a semiconductor such as P, Sb, Te, etc., or
an oxide thereof. The acicular ratio and the mean particle length
thereof are substantially the same as those of the ferromagnetic
iron oxide described above. However, it is not as preferred to use
ferromagnetic chromium dioxides since toxicity problems can
possibly occur.
The above-described ferromagnetic alloy particles have a
composition comprising more than about 75% by weight of a metal, at
least about 80% by weight of which metal is at least one kind of
ferromagnetic metal such as Fe, Co, Ni, Fe-Co, Fe-Ni, Co-Ni, and
Co-Ni-Fe, and with at least about 20% by weight, preferably 0.5 to
5% by weight, of the metal component being Al, Si, S, Se, Ti, V,
Cr, Mn, Cu, Zn, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au,
Hg, Pb, Bi, La, Ce, Pr, Nd, B, P, etc. As the case may be, a small
amount, e.g., less than 1% by weight of water and hydroxides may be
present.
The above described ferromagnetic alloy particles are fine
particles having a long axis of less than about 0.5 .mu.m.
The ferromagnetic particles in the inner layer of the present
invention are ferromagnetic iron oxide particles, and most of the
particles are maghemite. The ferromagnetic particles in the outer
layer of the present invention can be any of the above described
ferromagnetic particles, and maghemite, Co-containing maghemite,
Co-containing Berthollide iron oxides, CrO.sub.2, Fe-Co-Ni alloy,
or mixtures thereof are preferred.
The six known processes described below can be used to produce the
above-described ferromagnetic alloy particles.
(1) A process comprising decomposing organic acid salts of
ferromagnetic metals by heating and then reducing the products
using a reductive gas as described in, for example, Japanese Patent
Publication Nos. 11,412/'61; 22,230/'61; 14,809/'63; 3807/'64;
8026/'65; 8027/'65; 15,167/'65; 16,899/'65 (or U.S. Pat. No.
3,186,829); 12,096/'66; 14,818/'66 (or U.S. Pat. No. 3,190,748);
24,032/'67; 3221/'68; 22,394/'68; 29,268/'68; 4471/'69; 27,942/'69;
38,755/'71; 38,417/'72; 41,158/'72; 29,280/'73; and Japanese Patent
Application (OPI) Nos. 38,523/'72 and 88,599/'75.
(2) A process comprising reducing needle-like oxyhydroxides or
needle-like oxyhydroxides containing other metals, or further
needle-like iron oxides obtained from these oxyhydroxides as
described in, for example, Japanese Patent Publication Nos.
3862/'60; 11,520/'62; 20,335/'64; 20,939/'64; 24,833/'71;
29,706/'72; 30,477/'72 (or U.S. Pat. No. 3,598,568); 39,477/'72;
24,952/'73; and 7313/'74; Japanese Patent Application (OPI) Nos.
5057/'71 (or U.S. Pat. No. 3,634,063); 7153/'71; 79,153/'73;
82,395/'73; 97,738/'74; 24,799/'75; 51,796/'76; and 77,900/'76 and
U.S. Pat. Nos. 3,607,219; 3,607,220 and 3,702,270.
(3) A process comprising evaporating a ferromagnetic metal in a
low-pressure inert gas as described in, for example, Japanese
Patent Publication Nos. 25,620/'71; 4131/'72; 27,718/'72;
15,320/'74; 18,160/'74; Japanese Patent Application (OPI) Nos.
25,662/'73; 25,663/'73; 25,664/'73; 25,665/'73; 31,166/'73;
55,400/'73; and 81,092/'73.
(4) A process comprising pyrolyzing a metal carbonyl compound as
described in, for example, Japanese Patent Publication Nos.
1004/'64; 3415/'65; 16,868/'70; 26,799/'74; and U.S. Pat. Nos.
2,983,997; 3,172,776; 3,200,007; and 3,228,882.
(5) A process comprising electrolytically depositing ferromagnetic
metal particles using a mercury cathode and then separating the
deposits from the mercury as described in, for example, Japanese
Patent Publication Nos. 12,910/'60; 3860/'61; 5513/'61; 787/'64;
15,525/'64; 8123/'65; 9605/'65 (or U.S. Pat. No. 3,198,717) and
19,661/'70 (or U.S. Pat. No. 3,156,650); and U.S. Pat. No.
3,262,812.
(6) A process comprising reducing a salt of a ferromagnetic metal
by adding a reducing agent to a solution containing the salt as
described in, for example, Japanese Patent Publication Nos.
20,520/'63; 26,555/'63; 20,116/'68; 9869/'70; 14,934/'70; 7820/'72;
16,052/'72; 41,718/'72; and 41,719/'72 (or U.S. Pat. No.
3,607,218); Japanese Patent Application (OPI) Nos. 1353/'72 (or
U.S. Pat. No. 3,756,866); 1363/'72; 42,252/'72; 42,253/'72;
44,194/'73; 79,754/'73; 82,396/'73; 43,604/'74; 99,004/'74,
41,899/'74; 18,345/'75; 19,667/'75; 41,097/'75; 41,506/'75;
41,756/'75; 72,858/'75; 72,859/'75; 79,800/'75; 104,397/'75;
106,198/'75; and U.S. Pat. Nos. 3,206,338; 3,494,760; 3,535,104;
3,567,525; 3,661,556; 3,663,318; 3,669,643; 3,672,867; 3,726,664;
3,943,012; 3,966,510; 4,007,072; 4,009,111 and 4,020,236.
Ferromagnetic alloy particles prepared by processes (3) and (6)
described above are particularly effective in this invention.
The dual layer magnetic recording medium of this invention is
prepared by forming an inner magnetic recording layer on a
non-magnetic support by coating the inner magnetic recording layer
on the support followed by drying and further forming an outer
magnetic recording layer (surface magnetic recording layer) on the
inner magnetic recording layer using a similar step.
Coating compositions for magnetic recording layers used in this
invention are described in detail in, for example, Japanese Pat.
Nos. 15/'60; 26,784/'64; 186/'68; 28,043/'72; 28,045/'72;
28,046/'72; 28,048/'72; 31,455/'72; 11,162/'73; 21,331/'73 and
33,683/'73; U.S.S.R. Pat. No. 308,033; and U.S. Pat. Nos.
2,581,414; 2,855,156; 3,240,621; 3,526,598; 3,728,262; 3,790,407;
and 3,836,393.
The magnetic coating compositions described in these patterns are
mainly composed of ferromagnetic fine particles, binders and
solvents for coating but they contain, as the case may be,
additives such as dispersing agents, lubricants, abrasives,
antistatic agents, etc.
Thermoplastic resins, thermo-setting resins, reactive resins, and
mixtures of these resins can be used as binders in this
invention.
Suitable thermoplastic resins which can be used as binders in this
invention are resins having a softening point of lower than about
150.degree. C., a mean molecular weight of about 10,000 to 200,000,
and a degree of polymerization of about 200 to 2,000. Specific
examples of suitable resins are vinyl chloride-vinyl acetate
copolymers, vinyl chloride-vinylidene chloride copolymers, vinyl
chloride-acrylonitrile copolymers, acrylic acid ester-acrylonitrile
copolymers, acrylic acid ester-vinylidene chloride copolymers,
acrylic acid ester-styrene copolymers, methacrylic acid
ester-acrylonitrile copolymers, methacrylic acid ester-vinylidene
chloride copolymers, methacrylic acid ester-styrene copolymers,
urethane elastomers, polyvinyl fluoride, vinylidene
chloride-acrylonitrile copolymers, butadieneacrylonitrile
copolymers, polyamide resins, polyvinyl butyral, cellulose
derivatives (e.g., cellulose acetate butyrate, cellulose diacetate,
cellulose triacetate, cellulose propionate, nitrocellulose, etc.),
styrene-butadiene copolymers, polyester resins, various synthetic
rubber type thermoplastic resins (e.g., polybutadiene,
polychloroprene, polyisoprene, styrene-butadiene copolymers, etc.),
and mixtures thereof.
These thermoplastic resins are described in detail in, for example,
Japanese Patent Publication Nos. 6877/'62; 12,528/'64; 19,282/'64;
5349/'65; 20,907/'65; 9463/'66; 14,059/'66; 16,985/'66; 6248/'67;
11,621/'67; 4623/'68; 15,206/'68; 2889/'69; 17,947/'69; 18,232/'68;
14,020/'70; 14,500/'70; 18,573/'72; 22,063/'72; 22,064/'72;
22,068/'72; 22,069/'72; 22,070/'72; and 27,886/'73 and U.S. Pat.
Nos. 3,144,352; 3,419,420; 3,499,789; and 3,713,887.
Suitable thermo-setting resins or reactive resins which can be used
in this invention are those having a molecular weight of less than
200,000 as a coating liquid composition and after coating and
drying, the molecular weight of the resin becomes substantially
infinite due to condensation reactions, addition reactions, etc.
occurring. Also, resins which are not softened or melted before the
resins are decomposed are preferred. Specific examples of these
resins are phenol/formaldehyde novolak resins, phenol/formaldehyde
resole resins, phenol/furfural resins, xylene/formaldehyde resins,
urea resins, melamine resins, drying oil-modified alkyd resins,
carbolic acid resin-modified alkyd resins, maleic acid
resin-modified alkyd resins, unsaturated polyester resins, a
mixture of an epoxy resin and a hardening agent (e.g., a polyamine,
an acid anhydride, a polyamide resin, etc.), terminal isocyanate
polyester moisture-hardenable type resins, terminal isocyanate
polyether moisture-hardenable type resins, polyisocyanate
prepolymers (compounds having at least three isocyanate groups in
one molecule obtained by reaction of a diisocyanate and a low
molecular weight triol; trimers and tetramers of diisocyanates), a
resin of a polyisocyanate prepolymer and an active hydrogen
containing compound (e.g., a polyester polyol, a polyether polyol,
an acrylic acid copolymer, a maleic acid copolymer, a
2-hydroxyethyl methacrylate copolymer, a p-hydroxystyrene
copolymer, etc.), and mixtures thereof.
These resins are described in detail in, for example, Japanese
Patent Publication Nos. 8103/'64; 9779/'65; 7192/'66; 8016/'66;
14,275/'66; 18.179/'67; 12,081/'63; 28,023/'69; 14,501/'70;
24,902/'70; 13,103/'71; 22,065/'72; 22,066/'72; 22,067/'72;
22,072/'72; 22,073/'72; 28,045/'72; 28,048/'72; 28,922/'72; and
U.S. Pat. Nos. 3,144,353; 3,320,090; 3,437,510; 3,597,273;
3,781,210; and 3,781,211.
These binders may be used individually or as a combination thereof
and further, as the case may be, additives may be added to the
binder. A suitable weight ratio of the ferromagnetic particles and
the binder is in the range of about 10 to 400 parts by weight,
preferably 10 to 200 parts by weight, of the binder to 100 parts by
weight of the ferromagnetic fine particles.
The magnetic recording layer may further contain, in addition to
the above-described binders, ferromagnetic fine particles, etc.,
additives such as dispersing agents, lubricants, abrasives,
antistatic agents, etc.
Examples of dispersing agents which can be used in this invention
are fatty acids (e.g., of the formula R.sub.1 COOR, wherein R.sub.1
is an alkyl or alkenyl group having 11 to 17 carbon atoms) having
12 to 18 carbon atoms such as caprylic acid, capric acid, lauric
acid, myristic acid, palmitic acid, stearic acid, oleic acid,
elaidic acid, linoleic acid, linolenic acid, and stearolic acid;
metal salts, i.e., alkali metal salts (e.g., Li salts, K salts, and
Na salts, etc.) or the alkaline earth metal salts (Ma salts, Ca
salts, Ba salts, etc.) of these fatty acids; the
fluorine-containing esters of the above-described fatty acids;
amides of the above-described fatty acids; polyalkyleneoxide
alkyl-phosphoric acid esters; lecithin; trialkyl polyalkyleneoxy
quaternary ammonium salts (e.g., where the alkylene moiety has 1 to
5 carbon atoms, such as ethylene and propylene); and the like.
Moreover, higher alcohols having 12 or more carbon atoms and the
sulfuric acid esters thereof can also be used. A suitable amount of
the dispersing agent is usually about 0.5 to 20 parts by weight per
100 parts by weight of the binder used.
These dispersing agents are specifically disclosed in, for example,
Japanese Patent Publication Nos. 28,369/'64; 17,945/'69; 7441/'73;
15,001/'73; 15,002/'73; 16,363/'73; and 4121/'75 and U.S. Pat. Nos.
3,470,021 and 3,387,993.
Suitable lubricants which can be used in this invention include
silicone oils such as dialkyl polysiloxanes (with the alkyl moiety
having 1 to 3 carbon atoms), dialkoxypolysiloxanes (with the alkoxy
moiety having 1 to 4 carbon atoms),
monoalkylmonoalkoxypolysiloxanes (with the alkyl moiety having 1 to
5 carbon atoms and the alkoxy moiety having 1 to 4 carbon atoms),
phenylpolysiloxanes, fluoroalkylpolysiloxanes (with the alkyl
moiety having 1 to 5 carbon atoms), etc.; fine electrically
conductive particles such as graphite particles, etc.; fine
inorganic particles such as molybdenum disulfide, tungsten
disulfide, etc.; fine synthetic resin particles such as
polyethylene, polypropylene, ethylene-vinyl chloride copolymers,
polytetrafluoroethylene, etc.; alphaolefin polymers; unsaturated
aliphatic hydrocarbons which are liquid at room temperature
(compounds having n-olefin double bonds at the terminal carbon
atoms, having about 18 to about 24 carbon atoms); and fatty acid
esters comprising monocarboxylic fatty acids having 12 to 20 carbon
atoms and monohydric alcohols having 3 to 12 carbon atoms. A
suitable amount of these lubricants is usually about 0.2 to 20
parts by weight per 100 parts by weight of the binder.
These lubricants are disclosed in, for example, Japanese Patent
Publication Nos. 23,889/'68; 40,461/'71; 15,621/'72; 18,482/'72;
28,043/'72; 30,207/'72; 32,001/'72; 7442/'73; 14,247/'74; and
5042/'75; U.S. Pat. Nos. 3,470,021; 3,492,235; 3,497,411;
3,523,086; 3,625,760; 3,630,772; 3,634,253; 3,642,539; and
3,687,725; IBM Technical Disclosure Bulletin; Vol. 9, No. 7, 779
(December 1966); and Elektronik; No. 12, 380 (1961).
Materials generally used as abrasives, such as fused alumina,
silicon carbide, chromium oxide, corumdum, artificial corundum,
diamond, artificial diamond, garnet, emery (main components:
corundum and magnetite), etc., can be used as abrasives in this
invention. These abrasives used in this invention have a MoHs'
hardness of higher than about 5, and a mean particle size of about
0.05 to 5 microns, in particular 0.1 to 2 microns. These abrasives
are usually employed in an amount of about 0.5 to 20 parts by
weight per 100 parts by weight of the binder used.
These abrasives are described in, for example, Japanese Patent
Publication Nos. 18,572/'72; 15,003/'73; 15,004/'73 (or U.S. Pat.
No. 3,617,378); 39,402/'74; and 9401/'75; U.S. Pat. Nos. 3,007,807;
3,041,196; 3,293,066; 3,630,910; 3,687,725; British Pat. No.
1,145,349; West German Patent (DT-PS) Nos. 853,211 and
1,101,000.
Suitable antistatic agents which can be used in this invention are
fine electrically conductive particles such as carbon black, carbon
black graft polymers, etc.; natural surface active agents such as
saponin, etc.; nonionic surface active agents such as alkylene
oxide type surfactants, glycerin type surfactants, glycidol type
surfactants, etc.; cationic surface active agents such as higher
alkylamines, quaternary ammonium salts, pyridine and other
heterocyclic ring compounds, phosphoniums, sulfoniums, etc.;
anionic surface active agents containing an acid group such as a
carboxylic acid group, a sulfonic acid group, a phosphoric acid
group, a sulfuric acid ester group, a phosphoric acid ester group,
etc.; and amphoteric surface active agents such as aminoacids,
aminosulfonic acids, sulfuric acid esters or phosphoric acid esters
of aminoalcohols, etc.
Examples of surface active agents which can be used as antistatic
agents in this invention are disclosed in, for example, Japanese
Patent Publication Nos. 22,726/'71; 24,881/'72; 26,882/'72;
15,440/'73 and 26,761/'73; U.S. Pat. Nos. 2,271,623; 2,240,472;
2,288,226; 2,676,122; 2,676,924; 2,676,975; 2,691,566; 2,727,860;
2,730,498; 2,742,379; 2,739,891; 3,068,101; 23,158,484; 3,201,253;
3,210,191; 3,294,540; 3,415,649; 3,441,413; 3,442,654; 3,475,174;
and 3,545,974; West German Patent Application (OLS) 1,942,665; and
British Patent Nos. 1,077,317 and 1,198,450.
Furthermore, examples of suitable surface active agents are
described in Ryohei Oda, et al; Synthesis of Surface Active Agents
and Applications Thereof, Maki Shoten (1964); A. M. Schwartz &
J. W. Perry, Surface Active Agents, Interscience Publications
Incorporated (1958); J. P. Sisley, Encyclopedia of Surface Active
Agents; Vol. 2, Chemical Publishing Company (1964); and Kaimen
Kasseizai Binran (Handbook of Surface Agents), 6th Edition, Sangyo
Tosho K. K. (Dec. 20, 1966).
These surface active agents may be used individually or as a
mixture thereof and they can be used for other purposes such as for
improving the dispersion and magnetic characteristics, improving
the lubricating properties, and as a coating aid.
The magnetic recording layers of this invention are formed by
dispersing each of the components for the outer magnetic recording
layer and the inner magnetic recording layer followed by kneading
to provide a liquid coating composition for each, coating the
coating composition for the inner magnetic recording layer on a
non-magnetic support followed by drying, and then coating the
coating composition for the outer magnetic recording layer on the
inner layer followed by drying. During the period between the
coating of each of the coating compositions for the inner magnetic
recording layer and the outer magnetic recording layer and the
drying thereof, a treatment for orienting the ferromagnetic
particles in the magnetic recording layers can be employed and
further, after drying, a surface smoothening treatment may be
applied to these magnetic recording layers.
Suitable materials for the non-magnetic supports used in this
invention include polyesters such as polyethylene terephthalate,
polyethylene-2,6-naphthalate, etc.; polyolefins such as
polypropylene, etc.; cellulose derivatives such as cellulose
triacetate, cellulose diacetate, etc.; synthetic resins such as
polycarbonates, etc.; and metals such as aluminum alloys, copper
alloys, etc.
The support used in this invention may be in any form such as
films, tapes, sheets, etc. and the materials for the support may be
selected depending on the form employed.
A suitable thickness of these non-magnetic supports is about 2 to
50 .mu.m, preferably 3 to 25 .mu.m in the case of films, tapes, and
sheets and depending on the type of recorder used.
When the above-described support is used as the form of a film, a
tape, a sheet, a thin flexible disc, etc., the opposite side of the
support to the side having the magnetic recording layer thereon may
be coated with a so-called back coat for the purpose of prevention
of the generation of static charges, print through prevention,
prevention of the occurence of wow and flutter, etc.
Examples of back coat layers which can be employed in this
invention are described in, for example, U.S. Pat. Nos. 2,804,401;
3,293,066; 3,617,378; 3,062,676; 3,734,772; 3,476,596; 2,643,048;
2,803,556; 2,887,462; 2,923,642; 2,997,451; 3,007,892; 3,041,196;
3,115,420; 3,166,688 and 2,761,311.
Thus, the magnetic coating compositions in this invention are
prepared by kneading the ferromagnetic particles together with the
above-described binders, dispersing agents, lubricating agents,
abrasives, antistatic agents, solvents, etc.
The ferromagnetic particles and the above-described components can
be placed in a kneading machine simultaneously or successively, for
kneading. For example, a magnetic coating composition can be
prepared by adding ferromagnetic particles to a solvent containing
a dispersing agent and kneading the mixture for a definite period
of time.
Various kinds of kneading machines can be used for kneading and
dispersing the magnetic coating composition. For example, a
two-roll mill, a three-roll mill, a ball mill, a pebble mill, a
sand grinder, a Szegvari attriter, a high speed impeller, a high
speed stone mill, a high speed impact mill, a homogenizer, a
ultrasonic dispersing machine, etc.
Suitable techniques for kneading and dispersing which can be
employed in this invention are described in T. C. Patton; Paint
Flow and Pigment Dispersion, John Wiley & Sons Co. (1964) as
well as in U.S. Pat. Nos. 2,581,414 and 2,855,156.
Air doctor coating, blade coating, air knife coating, squeeze
coating, dip coating, reverse roll coating, transfer roll coating,
gravure coating, cast coating, kiss coating, spray coating, etc.,
can be employed and further other coating methods may also be
employed to coat the above-described magnetic recording layers on
the support. Descriptions of these coating methods are described in
detail in Coating Kogaku (Coating Engineering), pages 253-277,
Asakura Shoten (Mar. 20, 1971).
The dual layer magnetic recording medium of this invention is
prepared by forming two magnetic recording layers on a non-magnetic
support by repeating the steps of coating the magnetic recording
layer on the support using the above-described coating methods and
drying. Furthermore, the two magnetic recording layers may be
formed simultaneously using a simultaneous multi-layer coating as
described in, for example, Japanese Patent Application (OPI) Nos.
98,803/'73 (or West German Patent (DT-OS) No. 2,309,159) and
99,233/'73 (or West German Patent (DT-AS) No. 2,309,158).
Suitable organic solvents which can be used for coating the outer
magnetic recording layer and the inner magnetic recording layer are
ketones such as acetone, methyl ethyl ketone, methyl isobutyl
ketone, cyclohexanone, etc.; alcohols such as methanol, ethanol,
propanol, butanol, etc.; esters such as methyl acetate, ethyl
acetate, butyl acetate, ethyl lactate, glycol acetate monoethyl
ether, etc.; ethers and glycol ethers such as diethyl ether, glycol
dimethyl ether, glycol monoethyl ether, dioxane, etc.; aromatic
hydrocarbons such as benzene, toluene, xylene, etc.; and
chlorinated hydrocarbons such as methylene chloride, ethylene
chloride, carbon tetrachloride, chloroform, ethylenechlorohydrin,
dichlorobenzene, etc.
The magnetic recording layer formed on the support using the
above-described method is, if desired, subjected to a treatment to
orient the ferromagnetic particles as mentioned above, and then
dried. Also, if desired, the magnetic recording layer is subjected
to a surface smoothening treatment and further, the magnetic
recording medium thus formed is cut into the desired shape to
provide the magnetic recording medium of this invention. In
particular, it has further been found that by applying a surface
smoothening treatment to the surface of the magnetic recording
layer of this invention, a magnetic recording medium having a
smooth surface and excellent abrasion resistance can be
obtained.
In particular, it is preferred for the surface of the inner
magnetic recording layer to be subjected to a surface smoothening
treatment to provide a surface having a roughness of less than 0.2
S, an outer magnetic recording layer is coated on the inner layer,
and then, after drying, the surface of the outer magnetic recording
layer is subjected to a surface smoothening treatment to provide a
surface having a roughness of less than 0.2 S.
When an orientation treatment is employed, the magnetic field for
the orientation can be an alternating current magnetic field or a
direct current magnetic field of about 500 to 2,000 Gauss.
A suitable drying temperature for the magnetic recording layers is
about 50.degree. to 100.degree. C., preferably 70.degree. to
100.degree. C., more preferably 80.degree. to 90.degree. C.; a
suitable flow amount of air is about 1 to 5 kiloliters/m.sup.2,
preferably 2 to 3 kiloliters/m.sup.2 ; and a suitable drying period
of time is about 30 seconds to 10 minutes, preferably 1 to 5
minutes.
The orientation direction of the ferromagnetic particles is
determined depending on the use of the magnetic recording medium.
That is, in the case of an audio tape, a small-sized video tape, a
memory tape, etc., the direction of the orientation is parallel to
the lengthwise direction of the tape, while in the case of a
broadcasting video tape, the magnetic recording tape is oriented at
an angle of 30.degree. to 90.degree. to the lengthwise direction of
the tape.
Orientation methods for ferromagnetic particles are described in,
for example, U.S. Pat. Nos. 1,949,840; 2,796,359; 3,001,891;
3,172,776; 3,416,949; 3,473;960; and 3,681,138; and Japanese Patent
Publication Nos. 3427/'57; 28,368/'64; 23,624/'65; 23,625/'65;
13,181/'66; 13,043/'73 and 39,722/'73.
Furthermore, as described in West German Patent (DT-AS) No.
1,190,985, the direction of orientation may differ between the
outer magnetic recording layer and the inner magnetic recording
layer.
The above-described surface smoothening treatment for each magnetic
recording layer can be performed by calendering after drying or
using a smoothening sheet before drying.
In calendering, it is preferred for the smoothening to be performed
using a super calender method, wherein the magnetic recording tape
is passed through two rolls, e.g., a metal roll and a cotton roll
or a synthetic resin (e.g., nylon) roll. Super calender smoothening
is peferably performed under the conditions of an inter roll
pressure of about 25 to 50 kg/cm.sup.2, a temperature of about
35.degree. to 150.degree. C., and a speed of 5 to 120 meters/min.
If the temperature and the pressure are above the above-described
upper limits, the magnetic recording layers and the non-magnetic
supports are adversely affected. Also, if the treatment speed is
lower than about 5 meters/min., no surface smoothening effect is
obtained and if the speed is higher than about 120 meters/m.sup.2,
no benefits due to the operation are observed.
These surface smoothening treatments are described in, for example,
U.S. Pat. Nos. 2,688,567; 2,998,325; and 3,783,023; West German
Patent Application (OLS) No. 2,405,222; and Japanese Patent
Application (OPI) Nos. 53,631/'74 and 10,337/'75.
The invention is explained more specifically by reference to the
following comparison examples and examples. It is, however, to be
understood that the compositions, component ratios, the order of
operations, etc., shown therein can be changed or modified within
the scope of this invention. Therefore, the invention is not to be
construed as being limited to the examples shown below. Further,
all parts, percents, ratios and the like are by weight unless
otherwise indicated.
______________________________________ Comparison Example 1 (Single
layer magnetic recording tape) parts
______________________________________ Fine Ferromagnetic Particles
(as shown in Table 4) 100 Vinyl Chloride Resin (vinyl
chloride/vinylidene chloride ratio: 87 : 13 mol %; polymerization
degree : 400) 20 Acrylic Acid Ester/Acrylonitrile Copolymer (6 : 4
mol ratio) 15 Dibutyl Phthalate 2 Lecithin 1.5 Carbon Black (mean
particle size: 40 .mu.m) 0.5 Butyl Acetate 250
______________________________________
The above-described composition was mixed well to form a magnetic
coating composition, coated on a polyethylene phthalate support of
a thickness of 12 .mu.m at a specific dry thickness, and after
drying, the surface was subjected to a surface smoothening
treatment. The single layer magnetic recording tape thus obtained
was cut in a width of about 3.81 mm and then mounted in a
Philips-type tape cassette. Thus, Sample No. 1 and Sample No. 2
were prepared. The fine ferromagnetic particles used and the
properties thereof are shown in Table 4 below.
______________________________________ Comparison Example 2 parts
______________________________________ Fine Ferromagnetic Particles
(as shown in Table 4) 100 Vinyl Chloride Resin (vinyl
chloride/vinylidene chloride ratio: 87 : 13 mol%; polymerization
degree: 400) 20 Polyester Polyurethane (molecular weight: about
30,000: reaction product of diphenylmethane diiso- cyanate and a
polyester consisting of adipic acid with diethylene glycol and
butanediol) 10 Triisocyanate Compound (75% ethyl acetate solution
of the reaction product of 3 moles of toluene diiso- cyanate and 1
mole of trimethylol- propane, trade name, Desmodur L-75, made by
Bayer A. C.) 5 Dibutyl Phthalate 2 Lecithin 2 Butyl Acetate 250
______________________________________
The above-described composition was mixed well to provide a
magnetic coating composition for an inner magnetic recording layer
coated on a polyethylene terephthalate support of a thickness of 12
.mu.m at a specific dry thickness, and after drying, the surface of
the inner magnetic recording layer formed was subjected to a
surface smoothening treatment.
______________________________________ parts
______________________________________ Fine Ferromagnetic Particles
(as shown in Table 4) 100 Vinyl Chloride Resin (vinyl
chloride/vinylidene chloride ratio: 87 : 13 mol%; polymerization
degree: 400) 20 Acrylic Acid Ester-Acrylonitrile Copolymer (6 : 4
mol ratio) 15 Dibutyl Phthalate 2 Lecithin 1.5 Carbon Black (mean
particle size 40: .mu.m) 0.5 Butyl Acetate 250
______________________________________
The above-described composition was mixed well to form a magnetic
coating composition, coated on the above-described inner magnetic
recording layer at a specific dry thickness, and after drying, the
surface of the outer magnetic recording layer was subjected to a
surface smoothening treatment. The dual layer magnetic recording
medium thus prepared was cut into a width of about 3.81 mm and
mounted in a Philips-type tape cassette. Thus, Samples No. 3, No.
4, No. 5 and No. 6 were prepared. The fine ferromagnetic particles
used and the properties thereof are shown in Table 4 below.
TABLE 4
__________________________________________________________________________
Maximum Fine Residual Harmonic Ferro- Magnetic Opera- Frequency
Distor- Mag- Thick- magnetic Flux Densi- tion Sensi- Character- S/N
Dynamic tion Sample netic ness Particles Hc ty Br bias.sup.(6)
tivity.sup.(7) istics.sup.(8) MOL.sup.(9) Ratio.sup.(10)
Range.sup.(11) Factor.sup.(12) No. Layer (.mu.m) Type (oe) (Gauss)
(%) (dB) (dB) (dB) (dB) (dB) (%)
__________________________________________________________________________
No. 1 Single 5.0 .gamma.-Fe.sub.2 O.sub.3 310 1,600 102 .+-.0.0
+1.5 +2.5 51.5 72.0 1.40 Layer No. 2 Single 6.0 CrO.sub.2 480 1,900
160 -2.0 +3.2 +1.5 57.5 71.5 1.35 Layer Outer 2.4 .gamma.-Fe.sub.2
O.sub.3 390 1,400 Layer 105 +4.0 +2.0 +6.5 54.0 76.5 1.35 No. 3
Inner 3.6 .gamma. -Fe.sub.2 O.sub.3 270 1,800 Layer Outer 2.4
Co-FeO.sub.x 625 1,400 Layer 161 +3.5 +3.0 +7.5 61.0 77.5 1.35 No.
4 Inner 3.6 .gamma.-Fe.sub.2 O.sub.3 430 1,800 Layer Outer 2.0
CrO.sub.2 510 1,900 No. 5 Layer 106 +4.1 +3.5 +3.9 53.2 74.0 2.3
Inner 4.0 .gamma.-Fe.sub.2 O.sub.3 240 1,800 Layer Outer 2.0
Fe-Co-Ni 810 3,000 No. 6 Layer alloy 170 +3.3 +4.3 +4.7 58.8 74.6
2.2 Inner 4.0 .gamma.-Fe.sub.2 O.sub.3 385 1,800 Layer
__________________________________________________________________________
The values of the electromagnetic transformation characteristics
shown in Table 4 described above the Table 5 shown hereinafter are
based on Standard MTS-102 of the Society of Magnetic Tape
Industry.
(1) (Standard tape): QP-12 tape made by BASF A.G. is defined as the
standard tape.
(2) (Tape record for test): The tape record for testing is QP-12
tape recorded at output level when the residual magnetic flux of
the tape surface of a 333 Hz signal is 250 pwb per mm.
(3) (Normal bias current): The normal bias current is a bias
current giving an output of 0.5 dB lower than the maximum output of
a 4 KHz signal by the standard tape.
(4) (Normal output level): The normal output level is the output
level for testing when the first division (level controlling signal
of 333 Hz) of the test tape record is reproduced by the cassette
recorder for testing, and is also the output level when the
residual magnetic flux of the tape surface of a 333 Hz signal is
250 pwb per mm.
(5) (Normal input level): The normal input level is the input level
of a test cassette recorder which gives the normal output level
when a 333 Hz signal is recorded on the standard tape using the
normal bias current.
(6) (Operation bias): A 4 KHz signal is recorded on a sample tape
at a constant level corresponding to a level lower than the normal
input level while increasing the bias current, the bias current
when the reproduction output gives an output of 0.5 dB lower than
the maximum output is measured, and the difference from the normal
bias current is expressed as a percentage.
(7) (Sensitivity): A 333 Hz signal is recorded on a test magnetic
recording tape at the normal bias current and at an input of 20 dB
lower than the normal input level, the reproduction output level is
measured, and the difference from the level at 20 dB lower than the
normal output level is expressed in dB as the sensitivity.
(8) (Frequency characteristics): 333 Hz and 8 KHz signals are
recorded on a test magnetic recording tape at the normal bias
current and at an input of 20 dB lower than the normal input level,
the recorded signals are reproduced and the reproduction output
level of each signal is measured, and the relative ratio a of the
reproduction level of the 8 KHz signal to the reproducing level of
the 333 Hz signal is determined. Then, the same measurement is made
for the standard tape and the relative value a.sub.o of the
reproduction level of the 8 KHz signal to the reproduction level of
the 333 Hz signal is determined. Then, the difference d to the
standard value is obtained by the following relationship and is
expressed in dB:
(9) (Maximum non-distorted output (MOL): Recording and reproducing
are preformed at the normal bias current while increasing the input
level of the 333 Hz signal and the output level at which the third
harmonic wave component included in the reproduced output level
becomes 5% is determined. The difference between the output level
and the normal output level is expressed in dB.
(10) (S/N ratio): A 1 KHz signal is recorded on a test magnetic
recording tape at the normal bias and at the normal input level,
after continuing the recording while cutting the 1 KHz signal, the
recorded signal is reproduced, the reproduced output level of the 1
KHz signal and the noise output level on non-signal recording are
measured, and the difference between them is expressed in dB. The
reproduction was measured through the audio correction circuit of
JIS C 5542-1971 (magnetic recording tape test method).
Also, the dynamic range (D.R.) and the harmonic distortion factor
were measured as follows:
(11) (Dynamic range): The difference between the MOL of the test
tape and the level of the bias noise at 333 Hz is expressed in
dB.
(12) (Harmonic distortion factor): A 333 Hz signal is recorded on a
test tape at the normal bias current and the normal input level,
the signal is reproduced, and the harmonic distortion factor of the
third harmonic wave included in the reproduced signal is measured
and expressed as a percent.
In addition, for the samples of CrO.sub.2 type tapes (Samples No.
2, No. 4, No. 6, No. 8, No. 10, and No. 12), the sensitivity,
frequency characteristics, MOL, S/N ratio, dynamic range, and
harmonic distortion factor were evaluated at a bias current of
160%.
EXAMPLE 1
A composition the same as in that described for forming the inner
layer in Comparison Example 2 above, except that the fine
ferro-magnetic particles were replaced by the fine ferromagnetic
particles shown in Table 5 below, was mixed well to provide a
magnetic coating composition for an inner magnetic recording layer,
the coating composition was coated on a polyethylene terephthalate
support of a thickness of 12 .mu.m at a specific dry thickness
followed by drying, and the surface of the layer was subjected to a
surface smoothening treatment. Thereafter, the fine ferro-magnetic
particles for the outer magnetic recording layer as shown in Table
5 below were mixed at a mixing ratio (weight ratio) as shown in
Table 5 below and a composition the same as that for the outer
magnetic recording layer described in Comparison Example 2, using
the mixture of the fine ferro-magnetic particles in place of the
ferro-magnetic particles in Comparison Example 2, was mixed well to
provide a magnetic coating composition for the outer magnetic
recording layer, the coating composition was coated on the
above-described inner magnetic recording layer at a specific dry
thickness followed by drying, and the surface of the layer was
subjected to a surface smoothening treatment. The dual layer
magnetic recording medium thus obtained was cut into a width of
about 3.81 mm and was mounted in a Philips-type tape cassette.
Thus, Samples No. 7 to No. 11 were prepared using two kinds of fine
ferro-magnetic particles for the coating composition for the outer
magnetic recording layer and Sample No. 14 was prepared using three
kinds of fine ferromagnetic particles. The ferromagnetic particles
used and the properties thereof are shown in Table 5 below.
TABLE 5
__________________________________________________________________________
Maxi- mum Har- Residual Fre- monic Mix- Magnetic quency Dis- Fine
ing Flux Sen- Cha- tor- Ferromagnetic Ra- Den- Opera- si- racte-
S/N tion Mag- Thick- Particles tio sity tion ti- ris- Ra- Dynamic
Fac- Sample netic ness Hc (wt Br bias.sup.(6) vity.sup.(7)
tics.sup.(8) MOL.sup.(9) tio.sup.(10) Range.sup.(11) tor.sup.(12)
No. Layer (.mu.m) Peak Type (oe) %) (Gauss) (%) (dB) (dB) (dB) (dB)
(dB) (%)
__________________________________________________________________________
Outer Hc.sub.3 Fe-Co-Ni 610 50 Layer 2.0 alloy 2,200 102 +4.2 +3.3
+8.0 54.4 77.8 1.30 No. 7 Hc.sub.2 .gamma.-Fe.sub.2 O.sub.3 410 50
Inner Layer 4.0 Hc.sub.1 .gamma.-Fe.sub.2 O.sub.3 240 -- 1,800
Outer Hc.sub.3 Fe-Co-Ni 970 50 Layer 2.0 alloy 2,200 161 +3.7 +3.9
+8.5 61.0 78.0 1.30 No. 8 Hc.sub.2 Co-FeO.sub.x 650 50 Inner Layer
4.0 Hc.sub.1 .gamma.-Fe.sub.2 O.sub.3 385 -- 1,800 Outer Hc.sub.3
Fe-Co-Ni 720 70 Layer 1.0 alloy 2,200 105 +4.0 +3.6 +7.7 54.2 77.6
1.35 No. 9 Hc.sub.2 Co-FeO.sub.2 520 30 Inner Layer 5.0 Hc.sub.1
.gamma.-Fe.sub.2 O.sub.3 230 -- 1,800 Outer Hc.sub.3 Fe-Co-Ni 1,100
70 Layer 1.0 alloy 3,000 163 +3.6 +4.3 +9.0 61.1 77.7 1.35 No. 10
Hc.sub.2 Fe-Co-Ni 840 30 alloy Inner Layer 5.0 Hc.sub.1
.gamma.-Fe.sub.2 O.sub.3 375 -- 1,800 Outer Hc.sub.3 Fe-Co-Ni 830
20 Layer 3.0 alloy 2,200 161 +3.6 +3.7 +8.3 60.8 77.8 1.30 No. 11
Hc.sub.2 Co-FeO.sub.x 580 80 Inner Layer 3.0 Hc.sub.1
.gamma.-Fe.sub.2 O.sub.3 430 -- 1,800 Hc.sub.4 Fe-Co-Ni 675 35
Outer alloy Layer 1.5 Hc.sub.3 Co-FeO.sub.x 500 35 1,700 106 +4.1
+3.5 +8.2 54.4 78.0 1.30 No. 12 HC.sub.2 .gamma.-Fe.sub.2 O.sub.3
435 30 Inner Layer 4.5 Hc.sub.1 .gamma.-Fe.sub.2 O.sub.3 235 --
1,800
__________________________________________________________________________
The characteristics in Table 5 were measured in the same manner as
those in Table 4.
From the data shown in Table 4 and Table 5, the excellent effects
and advantages of this invention can be seen from a comparison of
the characteristics of the low-noise type comparison magnetic
recording tapes (Sample Nos. 1, 3 and 5) with the characteristics
of the low-noise type magnetic recording tapes of this invention
(Sample Nos. 7, 9 and 12) and from a comparison of the
characteristics of CrO.sub.2 type comparison magnetic recording
tapes (Samples Nos. 2, 4 and 6) with the characteristics of the
CrO.sub.2 type magnetic recording tapes of this invention (Sample
Nos. 8, 10 and 11).
As described above, the multilayer magnetic recording media of this
invention possess excellent advantages. Some of them are summarized
below.
(i) The sensitivity is high
(ii) The frequency characteristics are high;
(iii) There is no low sensitivity over the whole frequency range
and flat frequency characteristics are obtained with a standard
equalization;
(iv) The harmonic distortion is low and the MOL is very high;
(v) The S/N ratio is good; Furthermore,
(vi) Even though the magnetic recording medium of this invention
has two magnetic recording layers, the magnetic recording medium
provides the best quality of the tape using the conventional low
noise position and the conventional CrO.sub.2 position without the
need for a new tape select position.
While the invention has been described in detail and with reference
to specific emnbodiment thereof, it will be apparent to one skilled
in the art that various changes and modifications can be made
therein without departing from the spirit and scope thereof.
* * * * *